Concerning a heat-treating apparatus in which a strip is continuously heat-treated, various types of heat-treating apparatus are conventionally proposed. FIG. 1 is an arrangement view showing an example of the continuous strip heat-treating line. As shown in the view, a strip 11 is rewound by a payoff reel 1 and passes through a cleaning unit 2. Then the strip 11 passes through a heating zone 3, soaking zone 4, first quenching zone 5, heat-recuperating zone 6, over-aging treating zone 7, and second cooling zone 8. After that, the strip 11 is sent to a rolling mill 9 and then coiled by a tension reel 10.
In order to cool the strip in the first quenching zone 5 and the second cooling zone 8 in the above continuous strip heat-treating line, various cooling methods are conventionally proposed. When a general classification is made of these conventional cooling methods, the following three cooling methods are provided: a method of cooling a strip when a cooled roller comes into contact with the strip (Japanese Unexamined Patent Publication No. 59-143028); a method of cooling a strip when a cooling medium is directly blown against the strip (Japanese Unexamined Patent Publication No. 57-67134); and a method of cooling a strip when the strip is dipped in a cooling medium (Japanese Unexamined Patent Publication No. 54-162614).
In general, when the cooling zone is devised, these cooling methods are used singly or, alternatively, these cooling methods are used in combination with each other.
Next, referring to an example, the cooling method of cooling a strip by directly blowing a cooling medium against the strip will be explained as follows.
FIG. 2 is a cross-sectional view of the second cooling zone 8 taken on line X--X in FIG. 1. In this view, there is shown a means for cooling a strip by directly blowing a cooling medium against the strip. In the conventional cooling zone, the strip 11 is cooled as follows. The strip 11 is regarded as a flat shape, and cooling headers 12 are arranged in parallel with this flat strip 11. On the cooling headers 12, which are arranged in parallel with the strip, there are provided a plurality of cooling nozzles 13 which protrude perpendicularly to the cooling headers 12, and a cooling medium 14 is directly blown from the plurality of cooling nozzles 13 against the strip 11 so as cool the strip.
In the above construction, a plurality of cooling headers 12 are arranged in the direction of a vertical path in which the strip 11 is conveyed.
Water can be used as the cooling medium 14. In this case, water includes pure water, softened water, hard water, filtered water, clean water, fresh water, raw water and water into which an antioxidant is added. Also, gas can be used as the cooling medium 14. In this case, the gas includes atmospheric gas used in a furnace, inert gas such as argon, nonoxidizing atmospheric gas such as nitrogen, atmosphere or a mixed gas into which the above gases are mixed. The above are singly used, or alternatively the above are used in combination with each other.
As a special example of the cooling medium of liquid, there is proposed a method in which an organic solvent, the boiling point of which is high, or salt is used instead of water. In this connection, the methods of spray cooling and mist cooling are respectively defined as follows in this specification. When a strip is cooled by directly blowing a cooling medium against the strip, liquid such as water is singly used as the cooling medium. This cooling method is defined as spray cooling. When a strip is cooled by directly blowing a cooling medium against the strip, a mixture in which liquid such as water and gas are mixed with each other is used. This cooling method is defined as mist cooling.
When a strip passes in a vertical passage, it is warped in the longitudinal and the width direction because various stresses are given to the strip. FIG. 3 is a view showing a model of the cooling state in which a cooling medium is directly blown by the conventional means against the strip 11 which has been warped in the width direction as shown in FIG. 2.
When a cooling medium containing liquid such as water is directly blown against the strip 11 which has been warped in the width direction, the cooling medium 17 blown against the strip 11 locally concentrates at the center of the strip, in the width direction, on the concave side.
Further, in the vertical passage, the cooling medium which has concentrated upon the center of the strip in the width direction flows down along the strip in the longitudinal direction. Therefore, the center 15 of the strip in the width direction is overcooled.
FIG. 4 is a diagram showing an example of the temperature distribution in the width direction of the strip on the delivery side of the cooling zone in the case of mist cooling of the strip in the vertical passage of the conventional cooling method. As shown in the diagram, due to the phenomenon described before, the center 15 of the strip in the width direction is overcooled. Also, the edge portions of the strip in the width direction are overcooled.
In the edge portions 16 of the strip in the width direction, heat is removed from not only the back surface of the strip but also the edge surfaces of the strip. For this reason, the edge portions 16 of the strip in the width direction are overcooled.
When a strip is heat-treated in the continuous strip heat-treating line, various heat cycles are used according to the material of the strip to be manufactured. In general, as shown in FIG. 5, when a mild steel strip is manufactured, the following heat cycle is used. After the strip is heated to 700 to 900.degree. C. and soaked, it is cooled to 240 to 450.degree. C. in the first cooling zone 5 for over-aging, and then the strip is cooled to the room temperature in the second cooling zone 8.
When the strip is cooled in the respective cooling zones as described above, a temperature of the strip scatters. Due to the scatter of temperature, a material quality of the strip scatters.
Recently, there is an increasing demand of a so-called high-tension material. When a high-tension material is heat-treated in the above heat-treating line, the following problems may be encountered.
In the case of heat-treatment of the high-tension material, the temperature tends to vary in the width direction of the strip on the delivery side of the first quenching zone. Due to the above temperature variation, the mechanical strength of the strip varies, so that the material of the strip in the width direction varies. In order to solve the above problems, this defective portion of the strip caused in the mild steel strip or the high-tension material is conventionally removed by cutting off the defective portion on the delivery side of the continuous strip heat-treating line or in the finishing line.
However, the above method in which the defective portion is removed from the strip is disadvantageous as follows. The frequency of the occurrence of the defective portion scatters greatly. Therefore, it is necessary to manufacture the strip, the quantity of which is larger than a predetermined value. As a result, the production control becomes complicated. Further, it takes time and labor to detect the defective portion of the strip. When the defective portion is removed from the strip, the yield is deteriorated, and further the additional manufacturing process such as the finishing line, etc. is required. Therefore, the manufacturing cost is increased.